1. Field of the Invention
The present invention relates to a zoom lens unit, and an information device. Especially, the present invention relates to an improved zoom lens unit which is used as a photographing optical system, and an information device such as a digital still camera or a video camera or a silver salt camera or a portable information terminal device using the zoom lens unit as a photographing optical system.
2. Description of the Related Art
In recent years, a portable information terminal device provided with a digital camera or a camera which captures a still image or a video image by a CCD (charge-coupled device) image sensor or a CMOS (complementary metal-oxide semiconductor) image sensor, has become popular, instead of a still camera using a silver salt film as an imaging record medium, i.e. a silver salt camera.
The demand of users varies over a wide range. In particular, users always require a more compact imaging device which has a wide angle and a large magnification ratio.
From the viewpoint of compactness of a zoom lens unit which is used as a photographing optical system in the imaging device, it is necessary to shorten a total length of the zoom lens unit when changing magnification (when using), in particularly a total length of the zoom lens unit at a telephoto end.
In addition, from the viewpoint of a wide angle, it is preferable that a half-field angle at a wide-angle end be 38 degrees or more. The half-field angle of 38 degrees corresponds to a focal length of 28 mm in terms of a 35-mm silver salt camera (so-called Leica version).
Furthermore, from the viewpoint of a high magnification ratio, a zoom lens unit with a focus length of about 28-300 mm (about 10 times) in terms of a 35-mm silver salt camera is considered to be capable of responding to almost all general photographing conditions.
For example, as a zoom lens unit constituted of four lens groups, JP 2004-199000 A, JP 2005-326743 A, JP 2008-076493 A, JP 2008-096924 A, JP 2006-189598 A, JP 2008-026837 A and JP 2008-112013 A disclose a zoom lens unit which includes, in order from an object side to an image side, a first lens group having a positive refracting power; a second lens group having a negative refracting power; an aperture stop; a third lens group having a positive refracting power; and a fourth lens group having a positive refracting power, and when changing magnification from a wide-angle end to a telephoto end, an interval between the first lens group and the second lens group increases, an interval between the second lens group and the third lens group decreases, and an interval between the third lens group and the fourth lens group increases, the third lens group having at least two positive lenses and one negative lens.
However, in the zoom lens units disclosed in JP 2004-199000 A, JP 2005-326743 A, JP 2008-076493 A, and JP 2008-096924 A, since magnification ratios are about 2.8 to about 6.8 times, they are insufficient for the user's demand for the magnification ratio nowadays.
In addition, in the zoom lens unit disclosed in JP 2006-189598 A, a magnification ratio is about 15 times, which is large, however since a half-field angle is about 31 degrees, it is insufficient for the user's demand for the wide angle nowadays.
Further, in the zoom lens unit disclosed in JP 2008-026837 A, a magnification ratio is about 10 times, however, since a telephoto ratio is large, it is insufficient for compactness, and it is considered that there is still room for improvement in aberration correction.
In the zoom lens unit disclosed in JP 2008-112013A, although a magnification ratio is about 9.5 times, which is large, a half-field angle is about 40 degrees, and a telephoto ratio is comparatively small, which are favorable points, it is considered that there is still room for improvement in aberration correction of such as chromatic aberration.